Our long-term objective is to characterize the physiological processes that underlie involuntary muscle contractions, weakness and excessive fatiguability, major problems encountered in skeletal muscles paralyzed by injury or disease. In this proposal we will test the novel hypothesis that ongoing spontaneous motor unit activity, spasticity and muscle spasms preserve muscle contractile properties in the same way that exercise enhances muscle performance in uninjured people. In individuals with skeletal muscles paralyzed (not under voluntary control) or partially paralyzed by spinal cord injury, our Specific Aims are: 1) to characterize the patterns of involuntary muscle activity during spasms, contractions triggered by trivial - inputs such as light touch a few weeks post injury; 2) to compare the patterns, amount and duration of involuntary muscle activity recorded in the laboratory and over 24 hour periods with muscle contractile properties (strength, speed, fatigability) and measures of spasticity (eg. Ashworth scale); 3) to evaluate motor unit recruitment and rate modulation during spasms in relation to motor unit contractile properties measured by intraneural motor axon stimulation and/or spike-triggered averaging. Motoneuron excitability will be assessed from F- waves; 4) to describe the patterns of ongoing, spontaneous motor unit activity commonly seen in paralyzed muscles to discern whether it reflects changes in motoneuron properties and/or synaptic inputs; 5) to follow the time course over which involuntary muscle contractions develop and the changes in muscle strength, speed and fatigability that occur during the first year post injury (acute to chronic transition). Data from spinal cord injured people who do or do not take anti-spasm medication will be compared to that obtained from able-bodied controls during evoked and voluntary contractions. These data will improve our understanding of muscle spasms, the most debilitating aspect of the spasticity that is so prevalent after spinal cord injury and other neuromuscular disorders. The information obtained will also provide a rationale for the design of new rehabilitation that will aim to dampen involuntary muscle activity or to use it effectively to perform functional tasks.